Hydrostatic-transmission control system, especially for lift and other industrial vehicles

ABSTRACT

A hydrostatic-transmission control system for fork-lift trucks and like industrial vehicles has a pair of variable-displacement hydrostatic motors driving respective wheels on opposite sides of the vehicle and supplied by pump means having a control element for varying its displacement under the control of an operator. The prime mover, e.g. a gasoline engine or electric motor, drives the pump. The control system comprises means whereby, for low and average output speeds of the transmission consisting of a combination of pump means and motors, the control of the transmission ratio is a function only of the position of the aforementioned element whereas, for highest output speeds, the ratio is controlled at least in part independently of this element by a further parameter, e.g. prime-mover output.

FIELD OF THE INVENTION

The present invention relates to a control system for hydrostatictransmissions and, more particularly, to the control of hydraulicallypowered vehicles such as fork-lift trucks and other industrial vehiclesadapted to carry heavy loads or to travel at particularly low speeds forprecise placement of loads.

BACKGROUND OF THE INVENTION

Hydraulic transmissions and particularly hydrostatic transmissions arein widespread use for a number of purposes and generally comprise pumpmeans, consisting of one or more pumps, driven by a prime mover such asan internal-combustion engine or an electric motor, and motor means suchas one or more motors hydraulically connected with the pump means anddisplacing an output shaft or a number of such shafts.

The transmission ratio of such transmissions can be established byproviding the pump means with a control element which varies thedisplacement of the fluid per revolution. When the pump means is anaxial-piston pump, this control element can be a tiltable plate orcontrol disk which varies the stroke of the axial pistons of thecylinder barrel (see pages 113 ff. of FLUID POWER, U.S. GovernmentPrinting Office, Washington, DC, 1966).

Frequently the hydraulic motors are also of the axial-piston type(op.cit. pages 199 ff.) and can have a variable output depending uponthe position of the tiltable control element (usually the cylinderbarrel) as well. In this case the variable displacement can beconsidered in terms of the fluid displacement per output revolution ofthe input shaft.

Such transmissions can be set such that the output shaft rotates at lowspeeds with very high torque, at high speeds with low torque or at anyintermediate condition.

Transmissions of this type have received widespread application inindustrial vehicles adapted to transport or control large loads. Typicalof these applications is a fork-lift truck whose driving wheels may eachbe provided with a hydraulic motor forming part of a hydrostatictransmission of the type described. In these cases the hydraulictransmission has the advantage that it allows the vehicle to creep whilecarrying a relatively large load at a high position for precise stackingand to travel at high speeds when unloaded so as to move from place toplace at a storage or other industrial facility.

In all cases the prior-art control arrangements had a selectivelypositionable control element for the hydrostatic transmission whichwould set the transmission ratio, i.e. the speed ratio between the inputshaft of the pump means and the output shaft of the motor means asrequired by the vehicle operator, this element being coupled, forexample, to a pedal or lever at the driver position.

In most instances this element positively and directly established atransmission ratio so that for a given input shaft speed the outputshaft speed is always proportional to the setting of this element.

This has been found to be advantageous for many purposes and wasbelieved heretofore to be the most advantageous arrangement forfork-lift trucks and the like. For example, with a fork-lift truck, whena heavy load is lifted into a high position and must be set in placecarefully, this arrangement permitted the vehicle speed to bemeticulously controlled, even with nonlevel floors or travelingsurfaces, so that the vehicle could creep into a position independentlyof resistance to displacement of the vehicle. In general, therefore, itis of considerable advantage to have direct control of the transmissionratio by the operator for hydrostatic transmissions at low outputspeeds, especially for fork-lift trucks. Of course, the transmissionratio can be altered sharply when higher output speeds are required andthis was accomplished merely by resetting the operator-controlledelement.

In another conventional transmission arrangement, the transmission ratiowas made fully dependent upon the load at the output shaft, i.e. therequired output torque. This had the advantage that with increasingloading of the output shaft, the output speed was reduced so thatincreased torque was available and the power remained substantiallyconstant. This prevented overloading of the machine and even loading itto bring its output shaft to standstill. Control systems of this typeare commonly used for hydrodynamic transmission in road-travelautomotive vehicles although they have also been used with hydrostatictransmissions by way of suitable controls.

Systems of the latter type, however, have not heretofore been utilizedeffectively for industrial-plant vehicles because they do not permit thesensitive control of the speed and positions of the vehicle obtainablewith the direct transmission ratio control arrangement described above.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide animproved hydraulic transmission control system whereby disadvantages ofearlier arrangements can be avoided and sensitive transmission ratiocontrol can be obtained when necessary.

Another object of the invention is to provide an improved controlarrangement for hydrostatic transmissions, especially for industrialvehicles such as fork-lift trucks, enabling maximum utilization of theavailable power of the prime mover and transmission thereof.

Still another object of the invention is to provide a more efficientdrive system for fork-lift trucks and like vehicles.

It is yet another object of the invention to provide a controlarrangement for a hydrostatic transmission which improves thecharacteristics of the transmission and the drive arrangement.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained in accordance with the present invention in a controlarrangement for a hydrostatic transmission having a control elementdisplaceable by the operator and control means whereby at low or averageoutput-shaft speeds a transmission ratio between the input and outputshaft speeds is regulated exclusively as a function of the position ofthis element, and means whereby at output-shaft speeds greater thanthese, i.e. at the highest output-shaft speeds, the transmission ratiois controlled apart from this element in response to another operatingparameter, preferably an output parameter of the prime mover ortransmission.

When the principles of the present invention are applied to automotivevehicles, especially industrial vehicles of the fork-lift truck type,the pump means is preferably a variable-displacement hydrostatic axialpiston pump whose shaft is coupled to the prime mover and constitutesthe input shaft of the transmission. A swash plate or piston-strokecontrolling element is the means whereby the transmission ratio can beregulated between this input shaft and a pair of output shafts, drivenby respective axial-piston hydrostatic motors hydraulically connected tothe pump and further connected to the driving wheels of the vehicle. Theprime mover is an internal-combustion engine or an electric motor.

The present invention is based upon my recognition that thecharacteristics of direct transmission ratio control are important onlyat low speeds of the vehicle, for example for displacing a load of afork-lift truck. However, at higher speeds such precise control ofvehicle speed and transmission ratio is of no interest and it isadvantageous here to provide a more "automotive" or inertial controlwhereby the transmission setting is a function of the engine speed.

According to the present invention, the advantages of both transmissioncharacteristics are combined in that in the low speed range thetransmission ratio is directly controlled by the operator whereas in thehigh speed range it is controlled by another operating parameter of thevehicle system, for example the output speed of the engine.

The advantage of each of these two distinct transmission characteristicsis attained without loss of advantage of the other transmissioncharacteristic when either one is more desirable.

Conventional forklift trucks with internal combustion engines as primemovers and hydrostatic transmissions having their pump shaft connectedto the crankshaft of the engine, have a pedal directly coupled to theratio-changing element of the transmission and simultaneously entraininga speed-control element of the engine, i.e. the accelerator lever of thecarburetor.

With older and smaller forklift trucks the pedal directly sets thetransmission and can be depressed against a restoring force whichamounts to the resistance of the control element to displacement, i.e.the reaction of the pistons against displacement of the control element.In newer and larger forklift trucks, the pump is set by a servomechanismso that the speed-changing rate is limited by the rate at which theauxiliary servomechanism medium can be displaced in the servosystem. Ineither case the rate at which the ratio can be changed is limited to anacceptable degree. Consequently, for a given operating condition, themaximum power utilization can be described by a propulsion-forcehyperbola. This characteristic is so developed that, when the vehiclecarries its nominal load for displacement along a planar horizontalsurface, maximum possible acceleration is attained only when the driverpresses the pedal to the floor, i.e. "floors" the pedal. The entrainmentof the speed-adjusting member of the engine with the pedal is designedso that the crankshaft speed for any particular power is somewhatgreater than that which is required by the transmission. For climbing orfor displacing heavy trailers, an override control may be operated byhand for a still higher engine speed or the pedal must be furtherdisplaced to provide an acceleration.

The system of the present invention, as described above, should also bedistinguished from the characteristic of hydrodynamic converters andcorresponding controls for hydrostatic transmissions conventionally usedfor automotive purposes and wherein the vehicle speed is automaticallyadjusted in accordance with the resistance thereto. When the vehicle isoperated under a dragging load over long stretches with high speeds,these systems are highly advantageous since they compensate for changesin the contour between climbing, level and falling stretches of theroad. These systems, however, are not at all suitable for use withforklift trucks which frequently must travel with a raised heavy loadover poor surfaces with exact positioning of the load at a particularpoint at which the load may be set down or raised. Under theseconditions the vehicle must travel at very low speeds and be preciselycontrolled. The aforementioned system in which speed is a function oftravel resistance is totally ineffective under these conditions.

Thus, the arrangement of the present invention allows the vehicle tooperate under the most advantageous speed conditions, i.e. high speedsat which precise control is not needed and low speed at which suchcontrol is desirable, as required. Where precise control is desirablethere is direct control of the transmission ratio from the pedal,independent of the engine speed and hence a proportionality between thepedal position and the vehicle speed. The higher vehicle speeds are, forall practical purposes, an automatic control response to resistance totravel.

According to another feature of the invention, the further parameterwhich contributes to the control of the transmission ratio at higherspeeds of the vehicle is the output speed of the driving prime mover,e.g. an internal-combustion engine.

I have found it to be advantageous that not only the pump is avariable-displacement machine but each of the motors constitutes avariable-displacement machine as well and has a setting element, e.g. atiltable control plate or cylinder barrel, whose position determines thenumber of revolutions of the output shaft per unit volume of the fluidthroughput through the hydrostatic transmission.

According to an important feature of the invention, means is providedfor controlling each of these elements so that the transmission ratio isestablished solely by the setting element of the pump over a speed rangefrom standstill of the output shaft to a predetermined speed thereof,whereupon the control element of the pump is supplemented or replaced bythe control element of the motor for regulation of the transmissionratio in accordance with the aforementioned further parameter over aspeed range of the output shaft above this predetermined value.

Each of the hydrostatic motors of the transmission can be provided witha pressure-responsive servomechanism which has a piston defining in acylinder a responsive chamber which is connected by a conduit between aconstant-output auxiliary pump driven by the engine and a throttlelocation. A constant-volume auxiliary pump is a pump which has aconstant fluid output per revolution. The net output of the pump perunit time is thus a function of the engine speed so that the pressureahead of the throttle likewise is a function of the engine speed. Thisthrottle can be placed in effect automatically when the main pump, i.e.the pump of the transmission, has reached its greatestdisplacement-per-revolution setting.

According to still another feature of the invention, each of theservomechanisms operating the control element of the respectivehydrostatic motor has its compartment connected between the transmissionpump and a throttle having a variable flow cross section. The variableflow cross section of the throttle can be controlled by an elementindependently of the pump-control element, i.e. another lever or pedal,or an element operatively connected to the pump-control element, so thatthe throttle cross section starts to reduce and thereafter iscontinuously reduced when the transmission pump has been set into itsposition of greatest stroke volume per revolution. Thus furtheroperation of the control element for this pump in the direction tendingto increase the output speed of the transmission will not affect thevariable element of the pump but will only act upon the throttlepreviously mentioned.

I have found it to be desirable, with a system of the latter type andany arrangement in which the servomechanism for the hydraulic motor istapped from the main hydraulic medium network of the transmission, toprovide from each port of the transmission pump respective branchescontaining check values which run to a common junction, the check valvesrespectively permitting flow to respective ports from this junction.

A pressure relief valve is provided with its output side opening towardthis junction and its input side turned to one port of a changeovervalve which permits the higher pressure of the two main lines connectingthe transmission pump with the transmission motors to be delivered tothe pressure relief valve. This same port is advantageously connected toa fixed throttle which communicates via a pressure relief valve with thevariable throttle mentioned above. Each of the servomechanisms may haveanother compartment upon the servomechanism piston in a directionopposite that of the pressure in the first-mentioned compartment andconnected to a location between the fixed throttle and the firstpressure relief valve. The effective piston cross section in the secondcompartment can be substantially less than the effective piston crosssection of the first-mentioned compartments.

According to another feature of the invention, a controllable valve canbe provided in the line connecting the pressure source to thefirst-mentioned compartments of the servomechanisms and operated by thecontrol element for the transmission pump so that, upon resetting ofthis pump to a smaller stroke volume than its maximum, the firstcompartments of the servomechanisms are drained.

It has already been mentioned that a particularly desirable arrangementis obtained when the prime mover is an internal combustion engine andthe speed control for this engine is operatively connected with thetransmission ratio controller of the transmission. However, to anincreasing extent prime movers of fork-lift trucks are electric motorsand I have found that when the electric motor is a shunt wound motor thetransmission ratio control can be coupled with a field controller sothat in the high-speed setting the transmission ratio control comes intoplay to decrease the current through the field windings of the shuntwound motor.

As noted, it is especially advantageous with the present system toprovide a hydrostatic transmission in which both the pump and the motorsare of the variable-displacement type having respective control elementswhich regulate the strokes of the respective pistons. According to theinvention such a transmission is operated so that, from standstill to acertain output speed of the motor shafts, only the control element ofthe pump is displaced from its neutral or zero-displacement position toits maximum stroke volume position until the pump has achieved itsmaximum fluid displacement per revolution. Only thereafter is the motorelement shifted from its maximum fluid displacement position toward thesmallest permissible displacement position.

This mode of operation of the transmission has proved to be especiallyeffective for the precise control of low vehicle speeds and theoperation of the vehicle at high speeds by another parameter, e.g. theaccelerator position of an internal-combustion engine or a settingmember of an electric drive motor.

The pump is preferably directly controlled, i.e. the control memberoperated by the driver of the vehicle is coupled directly to thedisplacement-regulating element of the pump to obtain the desiredproportionality between the setting of the control member and thetransmission ratio for low speeds. The control elements of the motors,however, need be connected only indirectly with the control elementsince they may be operated, preferably automatically, by the otherparameter of the machine.

In many cases it is desired to operate below the maximum possibleacceleration or with less than maximum speed. In these conditions thehydraulic motor elements are operated only as desired. Thus, instead ofa load-regulating or power control which automatically maintains aconstant power, the secondary adjustment of the motor-control elementscan be achieved in a simple manner.

It is possible to control the secondary speed adjustment via an orificeso that, after the full angular displacement of the pump-control elementfor a given load condition, for example travel of a forklift truck withraised load along a planar surface, an approximately constant power ismaintained. Thus, there are two possibilities for exploitation of asingle throttle. Either the static pressure ahead of the throttleposition can be used to control the setting of the hydraulic motors, orthe flow of control fluid through the throttle constriction can be useddirectly to control the setting of the hydraulic motor. In the lattercase the rate of adjustment of the motor-control elements is limited bythe constriction.

Generally the hydraulic motor has only two basic positions, namely, aposition of maximum displacement over which the motor is operated untilthe pump-control element has reached its maximum angular position awayfrom the neutral, and the motor-control position corresponding tominimum stroke volume into which the motor is set once the pump hasachieved its maximum stroke position. The movement of the motor from thefirst to the second position takes place slowly.

If the operator wishes to maintain a constant vehicle speed between thespeed conditions at the two extremes of the motor settings, the controlis effected by operating the primary element, i.e. the pump control.

This poses no particular difficulty although it should be noted thatreadjustment of the pump-control element after it has been brought intoits extreme maximum-stroke position results in a shifting of themotor-control element from the secondary extreme position toward thefirst so that the control characteristic has a certain hysteresis.

The pump-control element may be operatively connected with a valve, forexample by a lever, so that when the pump-control element is at leastclose to its maximum stroke volume and is adjusted, the valve leading tothe servomotors of the hydraulic motors is shifted to drive the controlelements of these motors to a reduced angular position. The control ofthe motor elements by the hydraulic medium stream through theaforementioned throttle ensures that acceleration is never extreme andthat a reduced speed can be maintained by operation of the pump-controlelement. Of course, the valve can be closed even by the slightestdisplacement of the pump-control element if it is desired to switch outa response of the motor and vice versa.

With the combination of the present invention in which the primary side(pump) of the hydrostatic transmission is controlled, one can achievetrue constant-power control or at least a markedly simpler regulation ofthe vehicle speed with optimum characteristics both for low-speed andhigh-speed travel. The system utilizes the available power of theapparatus more fully and allows higher operating speeds, especially withforklift trucks. The pedal forces for control of the transmission can beheld relatively low and the pump and motor-control elements can bedirect, e.g. via levers, or with the aid of servomechanisms whereby theproportionality between the position of the actuating member (e.g. thedrive pedal) and the control element of the pump is maintained. Thedrive shaft of the pump may also be provided with an auxiliary pumpwhich can supply the servomechanisms of the hydraulic motors, preferablyvia an adjustable throttle which can be controlled from the pressure inthe transmission.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a hydraulic circuit diagram of a transmission according to theinvention;

FIG. 2 is a diagram of a portion of a control system illustrating theprinciples of the invention;

FIG. 3 shows another coupling in diagrammatic elevational view betweenthe control member and the system for regulating the hydraulic motors;

FIG. 4 is a diagram of still another arrangement embodying the presentinvention;

FIG. 5 shows a portion of the system of the invention as applied to anelectrically powered vehicle; and

FIG. 6 is a diagrammatic cross-sectional view of a changeover valve ofthe type used in accordance with the present invention.

SPECIFIC DESCRIPTION

For ease of understanding, the invention will be specifically describedas applied to a forklift industrial vehicle adapted to travel inwarehouses, dockyards and other industrial enclosures with the fork in araised or lowered position, with or without a load and powered by aprime mover which can be an internal-combustion engine or an electricmotor.

Where the prime mover is an electric motor it may be operated by abattery carried by the vehicle itself and recharged during periods ofnonuse of the vehicle. However, the system is applicable to otherhydraulically driven vehicles and hence the specific description asapplied to forklift vehicles should not be considered a limitation tothem although they are the preferred mode of realizing the invention.

Furthermore, when reference is made to a hydraulic pump, the pump may bea hydrostatic axial-piston pump of the type described in "FLUID POWER",supra. Such pumps have a swash plate for controlling the stroke of thepistons upon angular displacement of the swash plate from a neutralposition to angular extreme positions to either side of the neutralposition and corresponding to forward and reverse travel of the vehicle.The pump is driven by the prime mover.

The hydraulic motors described herein can be axial-piston motors asillustrated and discussed in "FLUID POWER" (supra) with variable strokeobtained by swinging the cylinder barrel relative to the output shafts.The output shafts are coupled to the driving wheels of the vehicleand/or to tracks for propelling the same.

Finally, by way of introduction, it will be noted that servomechanismsare sketchily described herein to displace the control elements of thepump or motors. In this case the more complex hydraulic servomechanismof "SERVOMECHANISM PRACTICE", McGraw-Hill Book Co., 1960, may be used,complete with pilot valves or pistons, follower pistons and the like.

Referring first to FIG. 1 it will be apparent that the basic elements ofa transmission, according to the invention, comprise a pump 1 driven, asrepresented by the dot-dash line 100, by an internal combustion engine101 whose carburetor 102 has its butterfly or accelerator control tiedvia a linkage 103 to an accelerator pedal 27. The pump 1 is reversibleas described and feeds hydraulic fluid via lines 104 and 105 to lines 2and 3 connected to one side of each of a pair of hydromotors 6 and 7.The other side of the pump is connected to lines 4 and 5 communicatingwith the opposite sides of the hydromotors 6 and 7 which have variabledisplacement, being provided with control elements represented at 6a and7a respectively. Each of these motors 6, 7 operates the drive wheel onone side of the vehicle.

A line 8 is connected to line 2 and line 9 is connected to line 4, thelines 8 and 9 extending to opposite inlet ports of a changeover valve 10best seen in FIG. 6.

The changeover valve as shown in FIG. 6 can comprise a housing 10aforming a cylindrical chamber 10b with frustoconical seats 10c and 10dat respective inlet ports 10e and 10 f. The seats 10c and 10d areengageable by ball-shaped closure members 10g and 10h held apart by aspring 10i within a tubular stem 10j carried by ball 10g. A stem 10kcarried by the other ball 10h is telescopingly received in the stem 10j.The housing 10a also has outlet ports 10m and 10n which are connected aswill be described in greater detail below.

When the pressure at inlet port 10e prevails over the pressure at inletport 10f, the ball 10g is thrust inward and the pressure of port 10e isdelivered to outlet ports 10m and 10n. When the pressure at port 10fprevails over the pressure at port 10g, member 10h is displaced inwardlyand this pressure is communicated to ports 10m and 10n.

Thus, the higher pressure prevailing in the lines 8 and 9 is applied tothe outlets 10m and 10n of the changeover valve and to the inlet of apressure-relief valve 11 which communicates with a line 12 tied by apair of check valves 13 and 14 to the lines 104 and 105. The checkvalves 13 and 14 prevent the pressure from the pump outlet, whicheverside it happens to be at a particular moment, from being applied to line12. These check valves, however, allow fluid from line 12 to be returnedto the intake side of the pump. Valve 11 thus effectively establishes athreshold pressure in line 15 to which the port 10n of the chamber 10 isconnected.

Line 15 terminates in a fixed-cross-section constriction 18 ahead ofanother pressure-relief valve 19 whose discharge side communicates witha variable-cross-section throttle 23 whose control member 26 is linkedto the accelerated pedal 27 of the engine 101.

A line 16 communicates between line 15, ahead of the throttle 18, andthe small-cross-section chambers 17 of a pair of servomotors 106, 107whose pistons 33 and 32, respectively, are connected to control elements6a and 7a of hydraulic motors 6 and 7.

The pistons 32 and 33 are stepped to have small-diameter ends in thecompartments 17 and large-diameter ends in cylinder bores 30 and 31 ofthe servomotors to define compartments 22 therein with larger effectivesurface areas than those of compartments 17.

A line 21 communicates between line 20, ahead of the adjustable throttle23, and the large-diameter compartments 22 via further throttles 108,109 whose flow cross sections regulate the rate at which fluid can befed to the compartments 22.

The control element 24 of the pump 1 is coupled by a lever network 110directly with a pedal 25 constituting the control member which is inturn displaceable by the operator.

Another arrangement (FIG. 2) can provide a small hydraulic (oil) streamfrom a feeding pump P to act against the spring force of the hydraulicmotor. In this case, the pump-control member 125 can operate the pumpelement 124 until it has reached its maximum displacement. Thereafterthe double-arm linkage 120 causes the valve 120c to be operated,introducing fluid to the servomotor 122 against its spring force.

When the control of the servomotors of the hydraulic motors is effectedby the static pressure ahead of a throttle in a circuit containing anauxiliary pump with a constant displacement, this static pressureincreases and thus raises the transmission ratio as soon as the speed ofthe engine and thus the auxiliary pump increases. Thus, when theaccelerator of the internal combustion engine is set to a higher valueand the engine is operating at higher power, the transmission isautomatically set to a higher speed of the output shafts of the motor.When the rotary speed of the engine is reduced, e.g. as a result ofoverloading the latter, the transmission is automatically set into asmaller ratio and hence relieves the loading of the internal combustionengine. The setting of the transmission pump does not need to be changedduring this operation.

Preferably, the transmission pump is actuated by a pedal 25 (FIG. 3)and/or a manually actuatable lever by the vehicle operator to establishthe transmission ratio at the desired level. The hydromotors can beoperated by the pressure in the transmission and preferably orespecially by the output speed of the engine. The control element of thehydromotors may be a valve which is switched into operation when thehydromotor control is desired.

The control element of the pump (125, FIG. 2) should be swingable ineither direction out of its neutral position for driving thetransmission in either direction and both forward and rearward travelmay utilize the characteristic of the invention whereby a prime moverparameter may be used to control the hydromotors when the pump elementis in its limiting operative position. However, this dual characteristicoperation is preferred for at least the forward movement.

The pump element may have a double-arm lever 125, each of whose arms isprovided with an abutment, the abutments being effective when theelement is in one or the other extreme position to actuate thehydromotor valve 120c always in the same sense. In this case it has beenfound to be advantageous to put the pilot valve or other input elementof the hydromotor servomechanism close to the actuator of the pump andthereby avoid linkages, setting errors or the like. This arrangementalso makes it possible to simply adjust the strokes of the variouselements so that directly upon termination of movement of thepump-control element, the movement of the motor-control element willcommence.

Referring again to FIG. 1 it will be seen that a displacement of thepump-control element 24 of pump 1, when the latter is driven by engine101, will result, for example, in the supply of fluid under pressure toline 104 and to the right side of each of the hydromotors 6 and 7 todrive the vehicle, for example, in the forward direction. A low pressureprevails in line 105 and a high pressure in line 104. As the pedal 25 isdisplaced still further, the deviation of control element 24 from itsneutral position increases, thereby increasing the pressure in line 104.This pressure is applied via line 8 to one input of the changeover valve10 and through this valve is applied to line 15 whence it is deliveredvia conduit 16 to the compartments 17 tending to hold the actuatingelements 6a and 7a in their maximum stroke positions.

If because of increased engine speed or greater resistance to forwardmovement, the pressure builds up in line 2 still further, fluid isforced through throttle 18 and operates the threshold pressure reliefvalve 19. The pressure delivered to line 20 is also a function of theflow cross-section provided by the throttle 23 which is linked with thepedal 27 operating the engine 101 i.e. the throttle 23 closes as enginespeed increases. Thus, the pressure delivered via line 21 tocompartments 22 is a function of the pressure threshold of valve 19, thepressure bled through throttle 18 and the flow cross-section of throttle23. Since the pressure at line 21 is invariably less than that at line15, but the effective cross-sectional areas of compartments 22 issubstantially greater than that of compartment 17, the pistons 32 and 33are subject to differential pressure displacement to the left to reducethe motor stroke and thereby the loading of the engine and increase theoutput speed.

When the control element 24 is swung in the opposite sense from itsneutral position, a similar operation ensures except that line 105 ispressurized and the hydromotors 6 and 7 are rotated in the oppositesense. In either case, excessive pressure buildup cannot occur becauseany excess pressure is bled back through pressure-relief valve 11 to theintake side of the pump.

FIG. 2 shows an arrangement in which a pedal 125 is connected to thecontrol element 124 of a pump 1 to operate the latter in eitherdirection. In either extreme position of the pump-control element, adouble-arm lever 120 is operated via its abutments 120a and 120b to opena valve 120c connected in series with the throttle 123 corresponding tothe throttle 23 previously described. In this embodiment the servomotor122 is supplied through the throttle 123 which is controlled by theaccelerator pedal 127 of the engine 101. Thus, instead of the staticpressure beyond the throttle 123 being effective to operate theservomotor of the hydraulic motor (e.g. motor 106), the control pressureis obtained by opening the valve 120c when the pump-control element 124has reached its limiting position.

In FIG. 3 I have shown an arrangement in which the pedal 25 is connectedvia a link 47 having a slot 111 with the control element 24 of thepump 1. In this embodiment, the control element 24 has a slide 49received in the slot 111 and normally urged by a spring 112 against theleft-hand end of this slot. This spring has a force which exceeds thenormal displacement force for the element 24 until its reaches itsextreme position.

A further link 48 is connected to link 47 and also has a slot 113 inwhich a slide 51 is received. The slide 51 is carried on the controlelement 114 of the throttle valve 23.

When the pedal 25 is displaced in a counterclockwise sense (arrow 115),therefore, the spring 112 will hold the slide 49 in its extreme positionuntil the control element 24 of pump 1 has reached its extremecounterclockwise position. At this point, further displacement of pedal25 will compress the spring 112 without further displacement of element24. At this point, the bottom edge 116 of the slot 113 engages the slide51 to enable the throttle 23 to be closed and increase the pressurebehind it. This pressure is communicated to the hydromotors via line 21as noted.

FIG. 4 shows that the engine 101 can drive the pump 201 (auxiliary pump)in addition to the transmission pump 1 which is not shown here. Theauxiliary pump has a constant displacement per revolution and forcesfluid to flow through a throttle 130 in a circulation path with areservoir 131. When the pedal 225 controlling the position of the pumpelement of the main pump 1 is displaced after the pump element hasreached its extreme operative position, e.g. via a lost-motionarrangement as shown in FIG. 3, it opens a valve 132 connecting alocation behind the throttle 130 with a servomotor 43 whose piston 43aoperates the control element 206a of the hydraulic motor 206 via a link44. The spring force against which this hydraulic force operates isrepresented by a spring 43b which can, of course, be the resilient forceof the hydraulic motor-control element resisting such displacement.

FIG. 5 shows a system in which a shunt-wound motor 300 constitutes theprime mover and is connected to the storage battery 301 of the forklifttruck by a controller 302 which reduces the field of the windings forincreasing speed. In this case, the motor 300 is connected to the pump 1and the control element 303 for the motor can be coupled with thethrottle 23 energized by this pump as described in FIG. 1 and not shownin further detail in FIG. 5. The output for the hydraulic motors istapped as in FIG. 1 from behind the throttle 23 via line 21.

Note that for a given hydraulic fluid flow rate (volume per unit time)for one of the hydraulic motors, the speed (RPM) of the motor increaseswhen the stroke volume per revolution is decreased.

I claim:
 1. A vehicle-drive system comprising:a prime mover; a pair ofdriven wheels for propelling said vehicle; a hydrostatic transmissionhaving variable-displacement pump means formed with an input shaftconnected to said prime mover, motor means hydraulically connected tosaid pump means and formed with at least one output shaft connected tosaid wheels, and a motor-displacement control element, said transmissionhaving a variable speed ratio between said input shaft and said outputshaft depending at least in part upon the position of apump-displacement control element for regulating the displacement perrevolution of said pump means; at least one operator-controlledactuating member; and control means operatively connecting said memberwith said pump-displacement control element, operatively connected tosaid motor-displacement control element, and effective at low to averagespeeds of said motor means and said output shaft for establishing saidratio solely as a function of said member and at higher speeds of saidmotor means and said output shaft controlling said ratio as a functionof the motor-displacement element as another operating parameter of thedrive system, said motor means comprising a pair of motors having avariable fluid displacement per revolution and respective controlelements for establishing the fluid displacement per revolution of saidmotors, each of said motors being formed with a hydraulic servomotoroperatively connected to the respective control element of the motor,said pump means including a reversible hydrostatic pump having a pair ofports, each of said motors having a pair of ports connected to the pairof ports of said pump, said control means comprising a changeover valvehaving inlet ports respectively connected to the ports of said pump andat least one output port, a first pressure-relief valve connectedbetween said output port and the ports of said pump by respective checkvalves, a first control line connected with the output port of saidchangeover valve and one side of each servomotor, a fixed-cross-sectionthrottle connected with said first line, a second pressure-relief valvedownstream of said fixed-cross-section throttle, avariable-cross-section throttle downstream of said secondpressure-relief valve, a second control line connected between saidvariable throttle and said second pressure-relief valve and opening intosaid servomotors on the opposite sides thereof, and means for varyingthe flow-cross-section of the variable throttle.
 2. A vehicle-drivesystem comprising:a prime mover; a pair of driven wheels for propellingsaid vehicle; a hydrostatic transmission having variable-displacementpump means formed with an input shaft connected to said prime mover,motor means hydraulically connected to said pump means and formed withat least one output shaft connected to said wheels, said transmissionhaving a variable speed ratio between said input shaft and said outputshaft depending at least in part upon the position of a control elementfor the displacement per revolution of said pump means; at least oneoperator-controlled actuating member and control means operativelyconnecting said member with said element and effective at low to averagespeeds of said motor means and said output shaft for establishing saidratio solely as a function of said member and at higher speeds of saidmotor means and said output shaft controlling said ratio as a functionof another operating parameter of the drive system, said motor meanscomprising a motor having a variable fluid displacement per revolutionand a respective control element for establishing the fluid displacementper revolution of said motor, said motor being formed with a hydraulicservomotor operatively connected to the control element of the motor,said control element of said motor being said other parameter, said pumpmeans including a reversible hydrostatic pump having a pair of ports,said motor having a pair of ports connected to the pair of ports of saidpump, said control means comprising a changeover valve having inletports respectively connected to the ports of said pump and at least oneoutput port, a first pressure-relief valve connected between said outputport and the ports of said pump by respective check valves, a firstcontrol line connected with the output port of said changeover valve andone side of said servomotor, a fixed-cross-section throttle connectedwith said first line, a second pressure-relief valve downstream of saidfixed-cross-section throttle, a variable-cross-section throttledownstream of said second pressure-relief valve, a second control lineconnected between said variable throttle and said second pressure-reliefvalve and opening into said servomotor on the opposite side thereof, andmeans for varying the flow-cross-section of the variable throttle.